Heat pumps have become increasing popular because of the energy efficiency in transferring rather than creating heat. A heat pump typically includes a compressor which circulates refrigerant through a first heat exchanger or condenser, through an expansion valve or opening, through a second heat exchanger or evaporator, and into a refrigerant storage device or accumulator. Vapor is withdrawn from the storage device for recirculation by the compressor. A heat pump can commonly be operated in either a heating or cooling mode by selective activation of a reversing valve.
In preferred operation, the refrigerant vapor is fully condensed to liquid at the exit of the condenser. In addition, the refrigerant is preferably fully vaporized at the exit of the evaporator as disclosed in Cochran U.S. Pat. Nos. 4,573,327 and 4,665,716, assigned to the assignee of the present invention. These patents further describe significant advances in the area of control of refrigerant flow using a charge control device in place of a conventional accumulator, and a float-type liquid control valve connected between the first and second heat exchangers to thereby enhance refrigerant flow and energy efficiency.
Air source heat pumps which exchange heat with ambient air have been most common because of their generally low initial cost. Another type of heat pump is the ground-coupled heat pump which transfers heat with the ground through a heat exchanger commonly called an earth tap. A ground-coupled heat pump is typically more efficient than an air source heat pump because the earth temperature may be more stable than ambient air.
Among the ground-coupled heat pumps are the direct expansion and closed loop type. The closed loop heat pump typically includes an intermediate fluid, such as an antifreeze solution, which is circulated between one or more buried conduits and a heat exchanger as disclosed in U.S Patent No. 4,325,228. In other words, an extra stage of heat exchange is required in the closed loop heat pump. The direct expansion heat pump circulates refrigerant directly through an earth tap heat exchanger and is more efficient than a closed loop heat pump. The term direct expansion derives from the direct expansion of the liquid refrigerant to vapor in the buried heat exchanger. In addition, the direct expansion heat pump does not require pumping of an intermediate fluid as does the closed loop heat pump. Unfortunately, a ground-coupled direct expansion heat pump may require a relatively large amount of refrigerant compared with an air-source heat pump or a closed loop heat pump.
Direct expansion heat pumps have used a U-shaped earth tap heat exchanger, including two parallel conduits joined in fluid communication at their lower ends, and wherein one conduit carries liquid and the other vapor. Unfortunately, in the cooling mode, heat may be transferred from the hot vapor conduit to the liquid conduit, particularly at the upper ends of the conduits. In other words, U-shaped tubes may be impractical for operation in the cooling mode because of thermal short circuiting between the side-by-side conduits.
Coaxial or concentric tubes for liquid and vapor refrigerant have also been used as disclosed, for example, in German Patent No. 3,203,526A. Typically a layer of insulation is provided between the concentric tubes. Unfortunately, coaxial tubes have the disadvantage that a relatively large amount of conduit material, usually copper, is required per square inch of earth contact as compared with the U-shaped tube configuration.
A significant portion of the cost of a ground-coupled heat pump may be attributed to the conventional earth tap heat exchanger. In particular, the thermally conductive material, such as copper, and the refrigerant quantity may desirably be reduced. Accordingly, greater acceptance of a ground-coupled heat pump may require advances in the area of the earth tap heat exchanger.